This invention addresses the current need for a compact, battery-operable, visible emission characterization system capable of determining the plume opacity of remote stationary emission sources. The opacity of an attenuating medium, such as a smoke plume, is defined as one minus the transmittance of that medium. The transmittance of an attenuating medium is the fraction of incident radiant energy that remains after passing through that medium. The plume opacity of an emission source indicates whether it meets regulatory compliance. The U.S. Environmental Protection Agency (EPA) has regulations that determine the maximum permissible opacity threshold of an emission source, and an opacity value beyond that threshold indicates non-compliance with those regulations. Both the U.S. EPA and the California Air Resources Board (CARB) are closely following the compact opacity measurement technology developed and disclosed under this present invention. Accurate measurement of this plume opacity by an automated handheld device is the goal of the present invention.
Methods established by the EPA for the remote measurement of the opacity of emissions from stationary sources include visual determination (via EPA Method 9), determination by lidar (via EPA Method 9, Alternate Method 1), and determination by camera (via EPA Alternative Method ALT-082, ASTM D7520-09). Of these methods, only the lidar method allows opacity determination during both daytime and nighttime hours because it contains its own light source or transmitter and it is not dependent on ambient light contrast conditions. The lidar method is real-time, provides post-observation data/evidence, and is theoretically the most accurate method. Currently, the most common method of determining opacity is by visual determination (EPA Method 9). This method introduces human bias since it relies on the human eye as a sensor, involves extensive labor costs to train personnel and repeat field certification every six months, and does not provide good post-observation data/evidence. While superior, lidar methods have not gained popularity or been in practice so far because existing lidar instruments are bulky, not easily transportable, not eye-safe, and consume significant power. This invention describes an instrument that incorporates state-of-the-art technologies as well as signal processing techniques resulting in a compact, easily portable, potentially eye-safe, energy efficient, battery-operable lidar system for the determination of opacity.
A basic lidar system consists of an optical transmitter, an optical receiver, and associated signal processing and control electronics. In a pulsed lidar system, the optical transmitter sends optical pulses in a collimated light path through the atmosphere towards a target of interest. A small fraction of that transmitted light is backscattered to the optical receiver by atmospheric constituents, particles, or objects within light path. The receiver collects the backscattered light onto a detector that converts that light into an electronic signal. The temporal response of this signal corresponds to a distance from the lidar system, since light must travel from the transmitter to a distance or range R and back to the receiver. The amplitude of the signal with an emission source in the light path compared to the amplitude of the signal with no emission source in the light path allows determination of the opacity of the emission source.
The present disclosure is directed to a novel opacity measurement system that is compact, handheld or hand portable and includes all hardware in one unit. More specifically, the present disclosure is directed to an opacity measurement system that can include a laser transmitter, a receiver with one or more detectors, the processing hardware, and power supply in one compact, hand portable unit. Historically, high-energy pulsed laser sources were large table-top units; however, with recent technological advances laser transmitters are much more efficient and can be very compact. By processing the lidar return signal with low-power analog circuitry or low-power, low-sample rate digital acquisition hardware, the need for power hungry, high-sample rate, signal acquisition hardware is eliminated. For example, U.S. Pat. No. 6,593,582 (Lee) and U.S. Pat. No. 7,741,618 (Lee) both prefer high pulse repetition frequencies (a few kHz) to improve the SNR for their application, both require a large diameter telescope (30 cm) for eye-safety, and both detection schemes uses photon counting which requires high power signal processing hardware. This type of system design criterion is common for those skilled in the art and cannot be powered by a lightweight battery which is contrary to the design criterion for the handheld system disclosed in this invention. The combination of efficient lidar system and low-power acquisition method allows significant reduction in power consumption of the system. With power consumption reduced by several orders of magnitude on both transmitter and signal processing systems, the entire opacity measurement system can be packaged in a compact, handheld or hand portable, battery-powered unit. This is the basis of this invention.